An analysis by the National Highway Traffic Safety Administration (NHTSA) of the fatality and injury statistics of passenger car occupants involved in side impact crashes led to the conclusion that the bulk of the crashes were vehicle to vehicle side impacts, rather than side impacts with stationary objects. It also showed that head injuries are the most frequent source of fatalities, followed by chest and abdominal injuries. A similar analysis of non-fatal side crashes showed that over 24,000 serious injuries occur annually. It is estimated that over 50 percent of these serious injuries result from impacts with the side interior and side hardware/armrests in passenger cars.
Automobile manufacturers have generally made structural changes to varying degrees and added energy absorbing padding in their passenger cars. However, it was found that structural modifications alone had little or no effect in reducing the injury measurements obtained in baseline vehicle tests, although there are studies that show that such modifications reduce the injury measurements on crash dummies. (These crash dummy tests are made with a moving deformable barrier simulating another vehicle striking the test vehicle.) NHTSA simulation studies have shown that structural stiffness and padding characteristics are important parameters that affect thoracic and pelvic injuries. A number of vehicle design characteristics such as the thickness of the door at occupant contact regions, stiffness of pillar-floor attachments, etc. also have a marked influence on dummy-measured injury parameters.
Even though researchers have differences of opinion on the mechanisms that produce injury in side crashes, the commonly held belief is that as the striking vehicle or barrier momentum is transferred to the target vehicle door, the door structure collapses inward with the inner panel striking the stationary occupant at a velocity that could range theoretically anywhere from the velocity change experienced by the center of gravity of the struck vehicle to the velocity of the striking vehicle or barrier. In most side crashes, this contact velocity of the door against the occupant's chest and pelvis in many production passenger cars is estimated to be as much as 12-15 kph (8-10 mph) less than the average lateral impact velocity of 50 kph (30 mph) of the striking vehicle. In order to minimize the thoracic injury potential, it is important to limit the contact velocity of the door interior against the occupant's chest. If the occupant comes in contact with the collapsing door as the door decelerates, one would expect the door-chest contact velocity to be lower. On the other hand, if the door strikes the occupant's chest as its velocity is being ramped up, and at or near the peak door velocity, the severity of the impact would be higher.
The lower contact velocity can be achieved by either locating the occupant as far away from the door as possible so that the door had begun decelerating prior to occupant contact, or by ensuring that the door offers enough resistance to sudden collapse so that it does not "punch" the occupant. Spacing the occupant away from the door to achieve a lower contact velocity is not a practical proposition in today's vehicles which have limited side crush space available in their basic design. Therefore, the only available practical means to achieve the objective of limiting the contact velocity, thereby transferring less energy to the occupant, is to make structural enhancements to slow the door down. Efficient management of the crash energy by dissipating it in the door would also reduce the chest and pelvic injury potential.
From the above discussion, it is clear that both structural improvements and providing energy absorbing materials such as padding to cushion the impact within the door are necessary to improve side impact safety performance in passenger cars. Some researchers have argued that the net effect of providing padding between the door and occupant for side impact protection could be an increased energy transfer to the occupant. They have argued that when padding is present, the occupant is contacted by the inner door surface earlier than when no padding is present. However, it is believed that it is possible to judiciously select the energy absorbing material so as to lower the energy transfer that takes place from the door to the occupant in a controlled fashion, and it is erroneous to theorize that any type of cushioning would necessarily result in higher thoracic injury levels because of earlier and hence increased energy transfer.
The design of a cushioning element for side impact protection depends on assumed impact conditions such as the impact velocity of the door and its mass, the contact area, the reaction surface characteristics, the available crush distance, and the surrounding structural compliance.
Customarily used energy absorbing materials can be grouped into three broad categories: resilient materials, quasi-resilient materials, and non-resilient materials. Resilient materials are designed to repeatedly absorb smaller amounts of energy over a reasonable period of time; quasi-resilient materials remain substantially resilient under small displacements, but under large distortions do not recover completely. Non-resilient materials, which do not recover, are generally suited for one-time absorption of very large amounts of energy. Open celled plastic foams, such as polyurethane and polyethylene foam, and rubberized materials fall into the resilient category, while fiberboard structures and plastic foams such as polyurethane, polystyrene, and polyethylene as well as rubbers of higher density than that of the resilient type are typical candidate quasi-resilient materials. Suitable non-resilient materials include paper and aluminum honeycomb sheets. Polyurethane and polyethylene foams are in both the resilient and quasi-resilient categories. As their densities increase they lose resiliency, hence the category a given foam is in depends on its density.
It would appear that for vehicle applications the most suitable are the quasi-resilient and the nonresilient type materials, the former for head impact protection in contacts against upper interior structures such as pillars, and the latter for side impact protection of the thorax. Both of the above types of materials can be easily formed into shapes and incorporated into various vehicle component designs. For those components which are most likely to be exposed to bumps and occupant contacts in everyday use, it would be preferable to use non-resilient materials. The initial peak stress and the energy absorbed in these materials are largely a function of overall material density. On the other hand, honeycomb is generally strain-rate insensitive at velocities of interest for vehicle applications. Therefore, the energy absorption characteristics of such materials are likely to approach those of ideal padding. Thus what is needed is a padding material or system which has a high initial energy absorption ability and which will continue to absorb energy at a high rate.